1. Field of the Invention
The invention relates generally to a variable valve timing system and a method for controlling the same, and, more specifically, to a variable valve timing system that uses an electric motor as an actuator and a method for controlling the same.
2. Description of the Related Art
A variable valve timing (VVT) system that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing system changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor.
With a variable valve timing system that hydraulically drives a camshaft, the variable valve timing control is sometimes not executed as accurately as it should be, in a cold environment or at the time of engine starting. Such inconvenience is caused because the hydraulic pressure used to drive the camshaft is insufficient or the response of the camshaft to the hydraulic control is slow in such occasions. To obviate such inconveniences, a variable valve timing system that drives a camshaft by means of an electric motor has been suggested, as described in, for example, Japanese Patent Application Publication No. JP-2004-156461 (JP-A-2004-156461). With a valve timing system described in JP-A-2004-156461, when the valve timing is changed to a value at or around the limit valve, to which the valve timing is able to be changed, the rate of change in the valve timing is restricted. Thus, the situation where a movable part of a variable phase mechanism collides with a stopper portion at a high speed is avoided.
A variable valve timing system is usually operated during the operation of a vehicle. However, even after a command to stop an engine is issued in response to an operation to turn off an ignition key, the variable valve timing system is sometimes operated to change the valve timing (the phase of a camshaft). More specifically, if the valve timing when a command to stop the engine is issued is not at the phase appropriate for the subsequent engine starting, the variable valve timing system changes the valve timing to facilitate the subsequent engine starting. For example, Japanese Patent Application Publication No. 2004-156508 (JP-A-2004-156508) describes a valve timing control system that changes the valve timing such that a camshaft is placed in an angular position appropriate for the subsequent engine starting. According to JP-A-2004-156508, the valve timing is changed by passing electric current through a hysteresis brake, which is an electromagnetically-controlled actuator, during a predetermined period after an ignition key is turned off.
With the valve timing control system described in JP-A-2004-156508, the valve timing control is executed such that the valve timing is changed to the valve timing appropriate for the subsequent engine starting. The valve timing is changed by passing electric current through the hysteresis brake during the predetermined period that starts when the ignition key is turned off. JP-A-2004-156508 describes that the valve timing control similar to that executed during the operation of an engine is continuously executed even after the ignition key is turned off. However, JP-A-2004-156508 has no description concerning a method for reliably changing the valve timing by a required amount by the time the engine is stopped (after the ignition key is turned off) in order to reliably causing the valve timing to match the target value when the engine is stopped (namely, at the subsequent engine starting time). Neither JP-A-2004-156461 nor JP-A-2003-328786 describes a method for reliably changing the valve timing by a required amount by the time the engine is stopped by executing the valve timing control in order to reliably cause the valve timing to match the target value when the engine is stopped.